1. Field of the Invention
This invention relates generally to a system and method for providing battery cell balancing and, more particularly, to a system and method that provides battery cell balancing using changes in state-of-charge (SOC) of each cell in the battery to determine the relative capacity of these cells.
2. Discussion of the Related Art
Electric vehicles are becoming more and more prevalent. These vehicles include hybrid vehicles, such as the extended range electric vehicles (EREV) that combine a battery and a main power source, such as an internal combustion engine, fuel cell systems, etc., and electric only vehicles, such as the battery electric vehicles (BEV). All of these types of electric vehicles employ a high voltage battery that includes a number of battery cells. These batteries can be different battery types, such as lithium ion, nickel metal hydride, lead acid, etc. A typical high voltage battery system for an electric vehicle may include a large number of battery cells or modules including several battery cells to meet the vehicle power and energy requirements. The battery system can include individual battery modules where each battery module may include a certain number of battery cells, such as twelve cells. The individual battery cells may be electrically coupled in series, or a series of cells may be electrically coupled in parallel, where a number of cells in the module are connected in series and each module is electrically coupled to the other modules in parallel. Different vehicle designs include different battery designs that employ various trade-offs and advantages for a particular application.
When a battery pack is relatively new, each cell in the battery pack typically operates at about the same level of performance, i.e., maximum charge or capacity. However, as the battery pack ages over time, each cell typically degrades in performance differently than the other cells, where the performance of the battery pack is limited by the performance of the lowest performing cell. Further, a battery cell or battery module in the battery pack may fail or may otherwise be limited in performance for other reasons, such as an internal short, loss of capacity, high resistance, high temperature, etc. If a failing cell or module can be identified, it may be possible to replace that particular cell or module without replacing the entire battery pack because it is generally cheaper to replace a single cell or group of cells than the whole battery pack. However, the new battery cell or module would not be at the same level of performance as the other battery cells in the battery pack, and therefore, the new cell needs to be matched in performance for proper operation of the battery.
Electric vehicles typically include cell balancing algorithms that control the charging of the battery so that the state-of-charge of the battery cells is about the same. Known cell balancing algorithms only look at cell state-of-charge, and not cell capacity, where the algorithm identifies the capacity of the cells as the same or nearly the same. When new battery cells are added to the battery pack, the cell balancing algorithm does not treat the new cells any different than the old cells so that during the charging sequence, the state-of-charge of the degraded cells will increase faster. This causes the cell balancing algorithm to discharge the higher state-of-charge cells faster, which are the cells that have the lower capacity, which causes them to be discharged too much during operation of a vehicle. Thus, improvements in cell balancing algorithms can be provided that will operate to extend the useful life of a battery pack.